The urgency of preserving our resources and our environment has stimulated intense efforts toward recycling manufactured products. Giving added impetus to these efforts are spiralling material costs and raw material shortages. Reclaiming some of these materials was once prohibitive economically, but now, in many cases, it is an attractive business.
A manufactured product that qualifies as prime candidate for new reclamation efforts is electrical wire and cable. The value of both copper and plastic insulation has increased sharply. Until recently, the copper content of some scrap wire and cable was reclaimed by burning away the plastic insulation. This practice has been terminated because of environmental considerations. Moreover, the plastic material has now enough intrinsic value to justify efforts to reclaim it as well as the copper.
Various methods of wire reclamation have been tried. These typically follow one of two general approaches. One of these is to chemically treat the scrap to dissolve one of the materials, ordinarily the insulation, leaving the other to be simply strained from the solution and dried. If the plastic insulation is dissolved, it can be reclaimed by chemical separation from the solvent.
The other approach is more mechanical in nature and relies on the difference in physical characteristics of the wire conductor and the insulation. The wire is cut into short lengths and mechanically agitated to free each metal wire segment from its insulation. The metal and plastic are then separated by a standard method such as flotation or gravity separation. An example of this technique is described and claimed in U.S. Pat. No. 3,670,969, issued June 20, 1972.
Interest continues in both of these kinds of reclamation processes. The solvent process is capable of recovering plastic in very pure form regardless of the mix of materials existing in the scrap. The mechanical process yields a mixture of polymers and plastisizers, the composition of which is normally controlled by first sorting the scrap. However, the mechanical kind of process is simpler and may prove to be less expensive.
This invention is an improvement of the mechanical separation method of U.S. Pat. No. 3,670,969. That method involves loading short segments of wire into a heated liquid medium and agitating the medium to release the metal from the plastic insulation. The technique appears to be based on empirical observations. The precise mechanics of the separation process are not apparent from the teachings of the patent. Those skilled in the art have speculated that a combination of the shearing forces of the agitated liquid medium on the wire seqments and of multiple impacts between wire segments and between the segments and the agitating hardware separates the metal from the plastic insulation.
It seems certain that the heated liquid plays an important role in heating the wire segments and that the heat facilitates separation. A possible explanation of the effect of heating is that it acts on the plastic insulation to allow extrusion strains in the plastic to relieve themselves, shrinking the insulation lengthwise and simultaneously increasing its diameters. This loosens the grip of the plastic on the conductor, so that the conductor can then be shaken or impacted out of the expanded segment of insulation, provided no sticking occurs. It is theorized that the liquid medium helps prevent sticking, as well as being an effective heat transfer medium for heating the scrap segments. As noted above, it also provides a mechanical medium for transmitting shearing forces and multiple impacts to the wire segments.
The foregoing explanation is a generalized opinion of how the prior art process operates. There is no quantitative theory available by which the effect of modifications in the prior art process can be predicted.